Fluorine treatment

The chemical resistance of PTFE is exceptional. There are no solvents and it is attacked at room temperature only by molten alkali metals and in some cases by fluorine. Treatment with a solution of sodium metal in liquid ammonia will sufficiently alter the surface of a PTFE sample to enable it to be cemented to other materials using epoxide resin adhesives. 

As far as can be ascertained, no performance standards exist for this product. In the absence of such standards, the existing standards for automotive vehicles were used as guidelines. By using the most stringent standard, the SHED test, a petrol permeation rate of approximately 3.3 g/m2 for 24 h at 40°C can be estimated. With a single-fluorination treatment a pipe already exceeds this standard with a steady state permeation rate of 1.7 g/m2 per 24 h at 50°C. Since it is a known fact that permeability increases drastically with a rise in temperature, a permeability ofless than 0.17 g/m2 per 24h at 30°C is expected for a single fluorination treatment. 

For both NH4,K-L and NH4, TMA-ft, the X-ray diffraction spectra (Figure Id, e) indicate that both zeolites retain most of their crystallinity after fluorine treatment and 500°C calcination. Except for some minor changes in peak intensity for fluorine-treated NH4,K-L, there is no evidence of peak intensity and position change as found for other treated zeolites. After 600°C calcination in air, a further loss of crystallinity is seen for fluorine-treated NH4,K-L. However, NH4,TMA-ft loses virtually all crystallinity. 

The spectrum for the 600°C-calcined fluorine-treated erionite sample shows substantial shifts in band positions, but band sharpening is less obvious. The bands at 1082, 792, 578, 470 and 438 cm- are shifted to 1098, 814, 585, 477 and 444 cm , respectively, after fluorine treatment and 600°c calcination. The large shifts observed are evidence of dealumination. The splitting of the 1082 cm-1 band into a doublet located at 1098 and 1085 cm-1 and some degree of band sharpening imply structure stabilization for fluorine treated erionite. 

Finally, I. R. spectra for fluorine-treated and then 500°C-calcined nh4,K-L and NH4,TMA-fl (Figures 2c and 2d) show an upshift of band positions, but loss of spectral resolution. Thus, the I. R. results indicate that the two zeolites undergo structural dealumination by fluorine treatment and subsequent calcination, but both show no evidence of structure stabilization. 

The adsorption of oxygen is used here as another measure of total crystallinity (Table III). LZ-105, H-zeolon, erionite, NH4,TMA-fi, NH4,K-L and NH4Y all show 10-20% loss in their total oxygen capacities. There are at least two explanations for such a reduction. First, the total pore volume is lowered because the total crystallinity after fluorination decreases slightly. Second, the fluorine treatment results in the entrapment of fluoride compounds such as MF, AIF3, A1F2(0H), A1F(0H)2, etc. in the pore systems. 

A simple shake test with aqueous 1 vol % n-butanol solution has been developed to test the selective adsorption of organic molecules over water molecules by a hydrophobic and organophilic adsorbent (26). Table V gives the results for severed, fluorine-treated zeolites and their untreated counterparts. The data clearly indicate that fluorine treatment of LZ-105, H-zeolon and NH4Y materials substantially increases their selectivity for n-butanol over water. 

As the data in Table VI show, the catalytic activity of the fluorinated zeolites can be either drastically increased or decreased depending on the treatment conditions and post-fluorination treatment. Varying the treatment conditions should allow the catalytic activity of a zeolite to be modified as desired. 

The behaviour of the fluorine treatment is much more complex. It is reasonable to think that the chemisorption of the fluorine on the carboxylic groups with formation of a strong dipole on the surface may play a very important role on the... 

Surface Modification of Inorganic Materials by Fluorination Treatments... 

Fluorination is effective not only for protecting surfaces from impurities but also for giving catalytic function and improving both the chemical and electrochemical characteristics of metal hydrides. For example, the surface of Mg2Ni created by a fluorination treatment was found to be effective as a catalyst for the catalytic generation of hydrogen from an aqueous alkaline solution of NaBH4 by hydrolysis (see Chapter 6.8). 

It forms a Ni-enriched top-surface above the sub-surface layer of Mgp2 that is followed by the ordinal phase of Mg2NiH4 by a fluorination treatment and the Mgp2 surface works effectively in hydrolysis to break the B—H bond to generate hydrogen (BH4" + 2H2O 4H2 + B02 ). 

A series of mischmetal nickel alloys are known as typical hydrogen-absorbing materials (see Chapter 6.3) and are used as the negative electrode in Ni-MH rechargeable batteries (see Chapter 8.4). By the fluorination treatment of mischmetal nickel alloys the electric conductivity is significantly improved by the replacement of La-oxide and La-hydroxide layers on the Ni-emiched Lap2 surface. The layer was also found to protect the surface firom KOH alkaline solution as electrolyte and to improve the long-term durability. 

Ordinary carbon fibers exhibit high thermal conductivity. If carbon fiber composites are used for thermal insulation, they must be modified. This could be done by disturbing the lattice with a fluorine treatment. 

Fig. 4.27a-f. Height images of membranes before and after fluorination treatment (scan size 2x2 im) a S, b SF, c M, d MF, e H, and f HF. Reprinted from [48]. Copyright 2002, with kind permission from Elsevier... 

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